From C. Ward Kischer, Ph.D., "Every human embryologist and every major textbook of Human Embryology states that fertilization marks the beginning of the life of the new individual human being. The reason why this is true is the following:
From the moment when the sperm makes contact with the oocyte, under conditions we have come to understand and describe as normal, all subsequent development to birth of a living newborn is a fait accompli. That is to say, after that initial contact of spermatozoon and oocyte there is no subsequent moment or stage which is held in arbitration or abeyance by the mother, or the embryo or fetus. Nor is a second contribution, a signal or trigger, needed from the male in order to continue and complete development to birth. Human development is a continuum in which so-called stages overlap and blend one into another. Indeed, all of life is contained within a time continuum. Thus, the beginning of a new life is exacted by the beginning of fertilization, the reproductive event which is the essence of life.
Herein lies the importance of distinguishing between the science of developmental biology and the science of Human Embryology. Within the science of Human Embryology, the continuum of life is more fully appreciated. The fact that development and developmental principles do not cease with birth becomes more fully realized. So, the continuum of human development does not cease until death, whenever that may occur, in utero or at 100 years of age."